A known shift controlling device of an automatic transmission of the vehicle determines a shift range based on a throttle valve opening of an engine depending on a vehicle speed and an operation amount of an accelerator pedal. In such configuration, a known vehicle weight estimating device estimates a vehicle weight which may be changed depending on, for example, a load or the number of passengers based on an acceleration and a driving force of the vehicle (e.g. Japanese Patent Laid-Open Publication No. 2002-340660). The estimated vehicle weight based on the acceleration and the driving force of the vehicle is used for determining the shift range of the automatic transmission. Such estimation is used for, for example, enhancing an effect of engine braking while the vehicle is running down a slope, and improving an accelerating performance while the vehicle is running up the slope. Thus, the shift range is changed depending on each running condition.
In the known vehicle weight estimating device, however, when the vehicle weight is estimated based on the acceleration obtained from the vehicle speed and the driving force obtained from a characteristic of a engine torque or a torque converter, an accuracy of the vehicle weight estimation may be greatly changed depending on an accuracy of each calculation of the acceleration and the driving force. Specifically, the accuracy of the vehicle weight estimation may be decreased because the acceleration and the driving force may be changed due to disturbance, a wheel may skid on a slippery road while acceleration, or the driving force may be changed due to a water temperature of the engine and an atmospheric pressure when the engine is driven. Thus, the accuracy of the vehicle weight estimation needs to be improved by preventing the fluctuation of the vehicle weight estimation due to aforementioned factors, and especially, there is a need to prevent the fluctuation of the vehicle weight estimation on the initial setting. The present invention therefore seeks to provide a vehicle weight estimating device for estimating the vehicle weight and improving the accuracy of the vehicle weight estimation on the initial setting.